Efficient traffic flows with connected vehicles

In the not-too-distant future, connected vehicles will be able to adapt to variable speed limits, increasing not only traffic efficiency but also road safety. Ellen Grumert has designed such a system during her doctoral project.

As we approach major cities along motorways, we are met by signs giving the current speed limit on variable message signs, sometimes 30, sometimes 50 or 80 km/hour. These variable speed limits are calculated by a mathematical algorithm that takes into consideration how many vehicles are on the road and how fast they are travelling, captured by data from stationary detectors along the road. In her licentiate thesis presented at the Division for Communications and Transport Systems at LiU, Ellen Grumert showed that using variable speed limits can increase traffic efficiency and reduce the risk of accidents. As part of her doctoral studies, she has worked at the department of traffic analytics at the Swedish National Road and Transport Research Institute, VTI.

Increasing number of connected vehicles

The work she presents in her doctoral thesis shows how to evaluate and develop this technology with the aid of computer microsimulations, in which individual vehicles are simulated in various traffic situations. The number of vehicles connected to the internet is increasing, and if it were possible to use information from these, the precision of the calculations could be significantly increased, and the number of fixed detectors needed could be decreased.

“The more vehicles that provide information about their position and speed, the more reliable will the calculations of the speed limit that gives the most efficient traffic flow along that particular stretch of road be,”

she says.

The information from the connected vehicles that are on a stretch of road makes it possible to estimate traffic with greater precision. It is also possible to reduce the amount of stationary detectors without reducing the performance of the estimation.

“Additionally, it will be possible to detect accidents or other obstacles faster. This information can be transmitted directly to the connected vehicles, and the drivers receive an early warning that something has happened.”

Towards driverless vehicles

The system also makes it possible to ensure that the vehicle is automatically slowed down to the variable speed limit that gives the highest traffic flow, which would be a step on the way towards the driverless vehicles of the future. In this case, stationary speed limit signs become unnecessary.

Part of Ellen Grumert’s thesis shows how to achieve better precision in calculations of the optimal speed limits. The factors include both the maximum allowed speed and the critical density, which measures how dense traffic can become on a road. A road with dense traffic that is close to the maximum capacity, or maybe even over it, is particularly sensitive to disruption. Motorways close to cities have very dense traffic and one small mistake by a driver can lead to major consequences, in the form of incidents and long traffic jams.

However, even if many vehicles are connected nowadays, it’s not always obvious to obtain data from them. Not only is there a need for a standard communication protocol, the vehicle manufacturers, or the drivers, must allow the vehicle to transmit data. A further question that must be considered is what happens if the system fails.

“They’ve been talking about international standards as long as I’ve been working in the field, but vehicle manufacturers, drivers and road operators are often reluctant to provide their data freely. The question of responsibility also remains to be answered,”

says Ellen Grumert.

Transport system of the future

She is, however, convinced that a variable and automatic speed limit system of the type described here will become a natural component in transport systems of the future.

“Since similar systems exist today, a natural next step is to take advantage of connected vehicles in such systems. It can provide huge benefits for both traffic efficiency and road safety, particularly on urban motorways close to larger cities.”

Ellen Grumert plans to continue to divide her time between work at VTI and research and teaching at LiU.